Update docs for `Size` and `RefCount` for `UsageData`

In 27294, `Size` and `RefCount` has been wrapped into `UsageData`
and is only exposed in `GET system/df`. Though the docs was not
updated in `docker_remote_api_v1.25.md`.

This fix updates the docs to refect the changes for `UsageData`.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/reference/api/docker_remote_api_v1.25.md

@@ -2663,8 +2663,10 @@ Return docker data usage information
                     "Mountpoint": "",
                     "Labels": null,
                     "Scope": "",
-                    "Size": 0,
-                    "RefCount": 0
+                    "UsageData": {
+                        "Size": 0,
+                        "RefCount": 0
+                    }
                 }
         ]
     }